I’m new to silversmithing and at the moment setting up my workshop.
I’ve bought a Sievert torch kit but can anybody tell me which burners
(with numbers) are best to use for soldering both small and large
pieces of silver and copper jewellery making, larger pieces are not
likely to be bigger than 3" diameter.

I use the 3940 for small stuff and the 3941 for bigger. Avoid the
little tip they sell for jewellery it is too hot and tricky to use. I
also use a much bigger tip for casting, I teach my students to hold
it like a pencil in their left hand, pointing down. That leaves your
right hand (if you are right handed) for fine control of tweezers
etc.

They are solidly built, a good valve and they will supply spares if
necessary. I have had mine 32 years. A little tip, if you lose the
washer that seals the burner into the bent tube on the handle they
are made of about 1.5mm sheet lead and are faster to make than
reorder.

It will not do everything, I use a mouth blown torch for really fine
work, and a water torch for platinum, but I can do 95% of what I need
to with it.

(it took a lot of sanding off the crud on the burners to get to the
number!!)